*nan-single*

This constant has the value of IEEE single-float NaN
(Not-a-Number). It is a legal floating-point value. In arithmetic
operations, it produces NaNs of appropriate format (following standard
floating-point coercion rules) in all cases. See the linked discussion
for more details.

NaNs like this one can result from normal arithmetic operations, such
zero divided by zero in a function with types declared and compiled so
comp:trust-declarations-switch is true,
but that same call run interpreted may instead signal an error.